The present invention relates to television receivers and, more particularly, to automatic gain control systems for such receivers.
Automatic gain control (AGC) circuits are well known in the art. A typical television receiver uses at least two AGC circuits in the tuner and associated intermediate frequency amplifiers in the front end of the receiver. One of the AGC circuits monitors the amplitude of the received video information signal and adjusts the gain of a radio frequency (RF) amplifier located in the tuner section. A second AGC circuit is associated with one of the intermediate frequency (IF) amplifiers which are connected to process the video information signal developed by the tuner. Generally, the AGC circuits are arranged such that a fixed gain is provided for low amplitude signals while stronger signals result in the activation of the AGC circuit to effectively reduce the gain of the amplifier. The first AGC circuit adjusts the gain of the RF amplifier in order to compensate for variations in the received signal level due to proximity of the transmitting station. The second AGC circuit is designed to assure that the amplitude of the video information signal is adjusted to a fixed value before being applied to the picture processing circuitry. For example, a typical arrangement adjusts the peak-to-peak amplitude of the synchronizing pulses associated with the received video signal to a predetermined value. The synchronizing pulses rather than the actual video information are sampled since the synchronizing pulses are presumed to be at a constant value. By adjusting the synchronizing pulses to the correct amplitude, the amplitude of the video information signal will necessarily follow.
One of the problems in the prior art systems is that the noise content of the video information signal also varies but not necessarily in correspondence with the video information content of a received signal. Accordingly, the signal to noise ratio in any AGC regulated video information signal may vary to such an extent that degradation of picture quality occurs even though the amplitude of the video signal is properly adjusted. Various proposals have been made to compensate for adverse signal to noise ratios in television receivers. U.S. Pat. No. 4,376,952 for example, discusses the prior art practice of depeaking a received signal by slight de-tuning of the receiver circuits. Such de-tuning is known to improve signal to noise ratio. However, such de-tuning also causes deterioration of the received video information in a received signal.
It is an object of the present invention to provide an improved automatic gain control system.
It is a further object of the present invention to provide an automatic gain control system which compensates for adverse signal to noise ratio signals.